The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. (See A. K. Varshneya, “Flat Glass”, Fundamentals of Inorganic Glasses (Academic Press Inc., Boston 1994), Chapter 20, section 4.2, pages 534-540.) Compared to other processes known in the art, for example, the float and slot-draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become highly important for the production of glass substrates that are used in the manufacture of liquid crystal displays (LCDs) and other substrates that require superior flatness and smoothness. The fusion process, and particularly the overflow downdraw fusion process, is discussed in commonly assigned U.S. Pat. Nos. 3,338,696 and 3,682,609 to Stuart Dockerty, and U.S. Pat. No. 3,437,470 to Overman, the teaching of which are incorporated herein by reference.
In the fusion process, the molten glass is fed into an isopipe and evenly flows over both sides to form a sheet of flat glass with pristine surfaces. The isopipe is designed to deliver the molten glass at a uniform flow rate, and the use of the isopipe and uniform flow rate are critical for the production of glass with uniform thickness. Due to high operating temperature and the gravitational load caused by isopipe itself and the molten glass, the isopipe sags over time with creep behavior. This causes the flow rate to change along the isopipe and affects the final glass quality. Methods of “sag control” have been described in commonly assigned US. Patent Application Publication Nos. 2003/0192349 A1 and 2004/0055338 A1; and also in Japanese Patent Application Publication Nos. 2004-315286 and 2004-315287. At the present time horizontal compression force is used to reduce the sag as is illustrated in FIGS. 1 and 2 from US. Patent Application Publication No. 2003/0192349 A1. However, as the isopipe becomes longer, higher compression forces are required and the implementation of such higher compression forces presents a challenging design procedure. In particular, as the size of the glass substrate being made using the fusion process and isopipe increases, the need for reducing isopipe sag to zero, or as near-zero as possible, becomes ever more important in order to maintain product quality and reduce costs.
Current isopipes behave like a simply-supported beam (see FIGS. 1 and 2). That is, both ends of the isopipe sit on pier blocks which prevent their vertical translation (FIG. 1). This type of the boundary condition allows the ends of the pipe to rotate and it results in non-zero slope at both ends. One method currently in use to correct the slope such that it remains zero, or as near-zero as possible, is to apply a compression force at the ends of the isopipe (see FIG. 2) to produce the counter-bending moment which can partly cancel out the bending-moment caused by the isopipe and the weight of the glass (that is, pipe “sag”). However, there are two disadvantages to this compression force remedy; namely:
the force is quite inefficient in creating a large amount of the bending moment since moment arm is very short; and
due to this inefficiency, substantial amount of force is required and this can result in potential buckling.
FIG. 1 is an illustration from US Patent application Publication No. 2002/019349 illustrating the construction of a fusion pipe (an isopipe) for use in the overflow drawdown fusion process. As illustrated in FIG. 1, the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in a refractory body 13 known as an “isopipe.” Once steady state operation has been achieved, molten glass passes from the supply pipe to the trough and then overflows the top of the trough on both sides, thus forming two sheets of glass that flow downward and then inward along the outer surfaces of the fusion pipe. The two sheets meet at the bottom or root 15 of the pipe, where they fuse together into a single sheet of glass (un-numbered). The single sheet is then fed to drawing equipment (represented schematically by arrows 17), which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root. The drawing equipment is located well downstream of the root so that the single sheet has cooled and become rigid before coming into contact with the equipment. As can be seen in FIG. 1, the outer surfaces of the final glass sheet do not contact any part of the outside surface of the fusion pipe during any part of the process. Rather, these surfaces only see the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the pipe, but those inner surfaces fuse together at the root of the pipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
As is evident from the foregoing, fusion pipe 13 is critical to the success of the fusion process. In particular, the dimensional stability of the fusion pipe is of great importance since changes in pipe geometry affect the overall success of the process. Unfortunately, the conditions under which the fusion pipe is used make it susceptible to dimensional changes. The fusion pipe must operate at elevated temperatures on the order of 1000° C. and above. Moreover, in the case of the overflow downdraw fusion process, the pipe must operate at these elevated temperatures while supporting its own weight as well as the weight of the molten glass overflowing its sides and in trough 11, and at least some tensional force that is transferred back to the pipe through the fused glass as it is being drawn. Depending on the width of the glass sheets that are to be produced, the pipe can have an unsupported length of 1.5 meters or more. Because of the high temperatures at which the process operates, the material of the pipe is susceptible to creep. Hence, the pipe sags steadily under gravity. Eventually the sag reaches a point where the quality and/or the dimensions of the finished glass are no longer within specifications and the pipe needs to be taken out of service and replaced. It is accordingly desirable to reduce the sag rate of the pipe, and thereby extend its useful life.
FIG. 2, also from US Patent application Publication No. 2002/019349, is a schematic drawing illustrating the use of off-center axial forces to control sag. In FIG. 2, pipe 13 is supported at its ends 23 by supports 21 and has a neutral axis 19. The neutral axis is that axis which does not elongate or contract as pipe 13 undergoes bending based on its mass distribution, its temperature distribution, and its material properties as a function of temperature. Put another way, the neutral axis is that axis which would not elongate or contract if pipe 13 were to undergo bending in the absence of axial forces F of FIG. 2 but with all other conditions the same. As shown in FIG. 2, in order to compensate for sag, axial forces F are applied horizontally to fusion pipe 13 at a distance H below neutral axis 19. Accordingly, the axial forces produce end moments of magnitude FH at the ends of the pipe. The sense of these moments is such that they reduce the tendency of the pipe to sag under the force of gravity. The moments produced by the axial forces will not eliminate all deformation of the pipe, but as illustrated by the comparative example presented below, a suitable choice of F and H will significantly prolong the useful life of the pipe. However, applying and maintaining the axial forces F is undesirable for reasons explained above
Thus, due to the difficulties encountered when compressive forces are use to prevent sag; there is a need for a better, and preferably simpler, method of preventing isopipe sag.